This invention relates to an apparatus for discharging material from a rotary mill that is used for grinding or comminution, and in which mill advantageously a grate and a pulp lifter are positioned upstream of a discharge opening in the proceeding direction of the material (i.e. the direction in which the material passes through the mill) and so installed in the interior of the mill that the grate and the pulp lifter are rotated with the rotation of the mill.
Pulp lifters in a mill for grinding or comminution of material transport the slurry passing through the apertures in the associated grate into the discharge opening of the mill. International Publication WO 98/01226 relates to a pulp lifter for a grate discharge mill. This pulp lifter comprises a plurality of chambers radially arranged to rotate against the downstream side of a vertical grate. A mill charge of mineral on the upstream side of the grate tumbles as the mill rotates. Water is fed to the mill and as the mineral is comminuted by the tumbling action, the fine particles and the water form a slurry in the interstices of the mineral. Some of the slurry passes through the apertures in the grate. Each chamber on the downstream side of the grate comprises a transitional compartment and a collection compartment. The transitional compartment has a wall that faces the grate, and this wall is formed with a plurality of apertures to enable the slurry to pass into the transitional compartment. During a portion of each rotation of the mill, each pulp lifter in turn passes against the mill charge on the upstream side of the grate and slurry passes through the grate to the transitional compartment. The pulp lifter is designed to enable slurry to pass from the transitional compartment to the collection compartment for subsequent discharge, but not back into the transitional compartment from the collection compartment. Thus, in the case of the practical implementation described in the WO publication, when the pulp lifter is at the bottom of its path of travel and slurry passes through the grate into the transitional compartment, the collection compartment is below the level of the transitional compartment.
The transitional compartment and the collection compartment disclosed in WO Publication WO 98/01226 form two substantially contiguous segments of the pulp lifter. Such segments can be separately divided into identical sections by a plate. The plates tilt slightly towards the grate side and are parallel with one another so that an area defined by one plate and one segment constitutes the transitional segment adjacent the grate, and an area defined by the other plate and the other segment constitutes the collection compartment, which is spaced from the face of the pulp lifter grate.
The pulp lifter described in International Publication WO 98/01226 is practical, when the mill is rotated at relatively low speed, i.e. below 75% of the critical speed of the mill. However, when the speed is increased higher, carryover of comminuted material in the collection compartment occurs and thus the effectiveness of grinding and comminution in the mill is decreased.
A disadvantage of many existing pulp lifters is that pebbles are able to accumulate in the pulp lifter and are recirculated as the mill rotates. The presence of a quantity of pebbles in the pulp lifter limits the space available for slurry and reduces the flow gradient through the grate, and may cause a slurry pool to be formed in the mill. In addition, many conventional pulp lifters are subject to disadvantage because of backflow from the pulp lifter through the grate. Thus, as soon as pulp lifter chamber fills with slurry, the flow gradient decreases and slurry may flow back through the grate as the pulp lifter rises.
The object of the present invention is to eliminate drawbacks of the prior art and to achieve a more effective apparatus for discharging material from a mill, which is used for grinding or comminution, even at the higher rotating speeds of the mill.